A digital audio/video data processing unit like a digital television decoder or “set top box” includes a main module gathering essential functions for the decryption of the received data, generally called deciphering unit or calculation module. The audio/video data entering the calculation module is encrypted with control words CW transmitted in a control message ECM stream associated with the audio/video data stream. The calculation module includes a particular circuit in charge of extracting the control words CW from the control messages ECM and decrypting them using keys made available by the processing unit and/or by a security module associated to said unit.
A security module is a device considered as tamper-proof containing different encryption/decryption keys, information used for identifying a user on a network and data which define rights acquired by the user for the reception of a broadcast audio/video data stream. The security module can be in different forms like a removable smart card inserted in a reader, an integrated circuit soldered onto a motherboard, a SIM type card (Subscriber Identity Module) as inserted in most mobile equipment.
The document EP1485857B1 describes a method for matching a decoder with a removable security module. The system formed by the decoder and the security module receives digital audio/video data encrypted by a control word and control messages ECM containing the encrypted control word. A first key is assigned to the decoder and a second key to the security module. These two keys form a unique pair in the broadcast network of the audio/video data. Only one key of the key pair can be chosen arbitrarily while the other is determined according to the first key in a way that the combination of these two keys conforms to a pairing key of the system, thus allowing to decrypt the control word.
The document EP1421789B1 describes a process of controlling access to encrypted data transmitted by an operator to a plurality of subscribers groups. Each group has a group key and each subscriber receives from the operator an operating key encrypted by the group key to decipher the transmitted data. The process consists of associating the operating key encrypted with the group key to a random value for generating a secret code. This code is transmitted via a management message EMM to the subscribers to calculate the operating key at the reception of the random value transmitted by control messages ECM. The process uses only one access control and it allows for dissuading the publication of the operating keys by making them dependent on the subscriber group.
The document EP1078524B1 describes a coupling or matching method in order to make a security module dependent on the host apparatus, in this case a Pay-TV decoder, and vice versa. The aim of this matching mechanism is to protect the communications between the security module and the decoder in order to prevent the capture, from the transmission channel, of the control words allowing deciphering the transmitted program data. The matching allows also to prevent the use of the security module with a foreign host apparatus or conversely. The solution uses a unique key or a unique key pair to encrypt and decrypt the data exchanged between the security module and the decoder. This unique key is maintained secret and unchanged during the whole life of the related devices. One or other of the connected devices can verify, at any moment, the validity of the matching parameters and take appropriate counter measures when a match is not found.
The document WO2006/040482 describes a method of recomposing a control word on the one hand by a security module and on the other hand by a decoder. Neither of the two devices can obtain alone the complete control word. The message including the two parts of the control word moreover contains two access conditions, one for the security module and the other for the decoder.